The distance between chassis and axle should not change for different loading states of the vehicle. For this reason, it is necessary to measure the height of the spring and to readjust the system by pumping or releasing air when there are deviations from the desired height. An air spring control is also required for optimal utilization of the axle load of a multi-axle vehicle.
Ultrasonic distance measurement in air is, for example, utilized for making fill-level measurements of vessels, to measure rooms of a dwelling, to make distance measurements when parking a motor vehicle, to make distance measurements in automatic focus cameras and the like.
German patent publication 3,423,602 discloses an arrangement for measuring the distance between the chassis and the axle of a vehicle utilizing an ultrasonic measuring system configured as a transmitter/receiver.
The advantage of utilizing ultrasonic sound within an air spring is that no turbulence of sound waves caused by the wind created by the movement of the vehicle is possible within the air spring.
On the other hand, the problem is present when making distance measurement within the air spring chamber that, in the air springs, pressure differences between 0 and approximately 20 bar and temperature ranges between -40.degree. C. and +120.degree. C. must be mastered. The speed of sound in a real gas, such as pumped-in air, is dependent to a great extent on pressure and temperature. For this reason, considerable errors result when basing the distance computation on a fixed pregiven speed of sound.
To avoid such errors, U.S. Pat. No. 4,798,369 discloses an ultrasonic air spring system wherein a compensation of the pressure and temperature dependency is provided by means of a computer circuit.
German patent publication 3,620,957 discloses an air spring having an ultrasonic pulse/echo system for making height measurements. An additional fixed target is proposed in order to cancel the effects of the changes in propagation speed of the pulses (the changes in propagation speed can occur because of air pressure, temperature and humidity changes in the interior of the resilient member of the air spring). In this way, a relative value for the propagation times is determined. It is not necessary to know the instantaneous speed of the sound to compute the vehicle elevation in this relative method.
The disadvantage with such a relative method while utilizing a reference distance (such a method is known in a comparable manner also for a telescope shock absorber as disclosed in German utility model registration G 87 02 817.4) is that the important variables of pressure and temperature cannot be explicitly displayed. The above-mentioned German utility model registration G 87 02 817.4 only discloses installing thermistors to compensate for temperature.
The spring pressure, which is of interest for the state of loading of the vehicle, the wheel load and the like, cannot be determined with any of the above-mentioned air spring measuring arrangements.